Cold welding crack is a very common defect in welding process. Particularly, when the high strength steel of low or middle alloy is welded, as the strength level increases, the propensity to form cold crack becomes greater. In order to prevent the generation of cold crack, preheating before welding and heat treatment after welding is usually required, thus, the greater the strength is, the higher the preheating temperature is, which leads to the complexity of the welding process and un-operability in some special cases, and endangers the safe reliability of welding structures, especially for large steel structures. In view of the fact that the industries such as petrification, high-rise buildings, bridges, and shipbuilding neither preheat the large high strength steel structure nor perform heat treatment after welding, the welding crack susceptibility index Pcm of steel is required to be as low as possible. Accordingly, a high strength steel plate having a low welding crack susceptibility has been developed in the metallurgy field.
A high strength steel having a low welding crack susceptibility, also called as CF steel, is a type of low-alloy high-strength steel of excellent welding property and low-temperature toughness, whose advantages are that preheating is not required before welding or a little preheating is required without the generation of crack, which mainly solves the welding procedure problem of large steel structures.
The way to reduce Pcm is to reduce the addition amount of carbon or alloying elements, however, to the high strength steel produced by quenching and tempering process, reducing the addition amount of carbon or alloying elements will inevitably lead to the reduction of the steel strength. The use of thermo-mechanical controlled rolling and cooling processes (TMCP) can overcome this defect. In addition, in comparison with the thermal refining process (quenching and tempering), the thermo-mechanical controlled rolling and cooling processes (TMCP) can refine the crystal grains, thus improving the low-temperature toughness of the steel.
At present, the alloying components of the steel having a low welding crack susceptibility produce by TMCP technology are typically Mn—Ni—Nb—Mo—Ti and Si—Mn—Cr—Mo—Ni—Cu—Nb—Ti—Al—B systems. For example, the chemical components of a low-alloy, high-strength steel produced by TMCP process disclosed in international publication WO99/05335 are as follows: (wt. %: percent by weight): C: 0.05-0.10 wt. %, Mn: 1.7-2.1 wt. %, Ni: 0.2-1.0 wt. %, Mo: 0.25-0.6 wt. %, Nb: 0.01-0.10 wt. %, Ti: 0.005-0.03 wt. %, P≦0.015 wt. %, S≦0.003 wt. %; for example, the chemical components of a superlow carbon bainitic steel disclosed in CN1521285 are as follows: (wt. %: percent by weight): C: 0.01-0.05 wt. %, Si: 0.05-0.5 wt. %, Mn: 1.0-2.2 wt. %, Ni: 0.0-1.0 wt. %, Mo: 0.0-0.5 wt. %, Cr: 0.0-0.7 wt. %, Cu: 0.0-1.8 wt. %, Nb: 0.015-0.070 wt. %, Ti: 0.005-0.03 wt. %, B: 0.0005-0.005 wt. %, Al: 0.015-0.07 wt. %.
The alloying elements of the above two disclosed steels are designed as Mn—Ni—Nb—Mo—Ti and Si—Mn—Cr—Mo—Ni—Cu—Nb—Ti—Al—B systems, respectively. Since Mo and Ni are both noble metals, the production costs of these types of steel plates are relatively high from the point view of the type and the total addition amount of the added alloying elements. In addition, they both use tempering heat treatment, which increases the manufacture procedures of the steel plate and enhances the production cost of the steel plate, and their Pcm values are relatively high, which has an adverse impact on welding performance.
In order to solve the above problems, the present inventors use the steel of a Si—Mn—Nb—Mo—V—Ti—Al—B system, and the present inventors design a superfine bainite battened steel plate having a low welding crack susceptibility and a yield strength of 800 MPa by use of the reinforcement effect of V, and thermo-mechanical controlled rolling and cooling processes without thermal refining, the resultant steel plate having excellent low-temperature toughness and weldability.
Contents of the Invention
The object of the present invention is to provide a steel plate having a low welding crack susceptibility and a yield strength of 800 MPa.
The other object of the present invention is to provide a manufacture method for the steel plate having a low welding crack susceptibility.
In the first aspect of the present invention, it is provided a steel plate having a low welding crack susceptibility and a yield strength of 800 MPa, which comprises the following chemical components (wt. %: percent by weight): C: 0.03-0.08 wt. %, Si: 0.05-0.70 wt. %, Mn: 1.30-2.20 wt. %, Mo: 0.10-0.30 wt. %, Nb: 0.03-0.10 wt. %, V: 0.03-0.45 wt. %, Ti: 0.002-0.040 wt. %, Al: 0.02-0.04 wt. %, B: 0.0010-0.0020 wt. %, the balance being Fe and unavoidable impurities, and the welding crack susceptibility index meets the following formula: Pcm≦0.20%.
The steel plate with a low welding crack susceptibility has a superfine bainite battened structure.
The susceptibility index Pcm to welding cracks of the steel plate with a low welding crack susceptibility can be determined according to the following formula:Pcm(%)=C+Si/30+Ni/60+(Mn+Cr+Cu)/20+Mo/15+V/10+5B.
The welding crack susceptibility index Pcm is an index reflecting the welding cold crack prospensity of steel. The smaller the Pcm is, the better the weldability is, while the larger the Pcm is, the worse the weldability is. Good weldability refers to a steel which uneasily generates welding crack upon welding, while poor weldability refers to a steel which easily generates crack. In order to avoid the generation of crack, preheating of the steel is required before welding, and the better the weldability is, the lower the required preheating temperature is, contrarily, the higher preheating temperature is required. According to the stipulations of the Chinese ferrous metallurgy industry standards YB/T 4137-2005, Pcm value for the steel of trademark Q800CF should be lower than 0.28%. The Pcm of the steel plate with a low welding crack susceptibility of the present invention is lower than 0.20%, which accords with the stipulations of the above standard and has an excellent welding property.
The chemical components of the steel plate with a low welding crack susceptibility and a yield strength of 800 Mpa is described in detail in the following contents.
C: Enlarging an austenitic area. C in a supersaturated ferrite structure formed in the quenching process can increase the intensity of the steel. However, C has an adverse impact on welding performance. The higher the content of C is, the poorer the welding performance is. As to a bainitic steel produced by TMCP process, the lower the content of C is, the better the toughness is, and a lower C content can produce a thicker steel plate of higher toughness, and a superfine bainitic matrix structure containing a high dislocation density can be obtained. Therefore, the content of C in the present invention is controlled at 0.03 to 0.08 wt %.
Si: Not forming carbide in the steel, but existing in the bainite, ferrite or austenite in the form of a solid solution, which can improve the intensity of the bainite, ferrite or austenite in the steel, and the solution strengthening effect of Si is stronger than that of Mn, Nb, Cr, W, Mo and V. Si can also reduce the diffusion velocity of carbon in the austenite, and makes the ferrite and pearlite C curve in the CCT curve move rightwards, thus facilitating the forming of bainite structure in the continuous cooling process. In the inventive steel, no more than 0.70 wt % of Si is added, which is favourable to improve the matching relation of intensity and toughness of the steel.
Mo: A ferritizing element, which reduces the austenitic area. Mo, solid solved in austenite and ferrite, can increase the intensity of the steel, improve the hardenability of the steel and prevent temper brittleness. Since the present invention does not need the treatment of thermal refining, only no more than 0.30 wt % of Mo, which is a very expensive element, is added to achieve the purpose of reducing the cost.
Nb: In the present invention, a relatively high amount of Nb is added in order to realize two purposes, in which one purpose is to refine crystal grains and increase the thickness of the steel plate, and the other purpose is to enhance the non-recrystallization temperature of the steel and facilitates the use of relatively high finish rolling temperature in the rolling process, thus accelerating the rolling speed and increasing the production efficiency. In addition, since Nb strengthens the grain refining effect, thicker steel plate can be produced. In the present invention, 0.03-0.10 wt. % of Nb is added to give consideration to the solution strengthening effect and the fine grain strengthening effect of Nb.
V: A ferritic formation element, which reduces austenitic area significantly. V dissolved in an austenite at a high temperature can improve the hardenability of the steel. The carbide of V, i.e. V4C3 in the steel is relatively stable, and can inhibit the movement of the grain boundary and the growth of the crystal grains. V can refine the as cast structure of welding metal, reduce the overheating sensitivity of the heat affected zone, and prevent the excessive growth and coarsening of the grains near the fusion line in the heat affected zone, which is favorable to the welding performance. In the present invention, 0.03-0.45 wt. % of V is added to improve the intensity of the steel greatly. V and Cu can both play a role of precipitation strengthening in the steel, however, in comparison with Cu, only a minute quantity of V is added to achieve the same precipitation strengthening effect. In addition, since Cu tends to induce the grain boundary cracks in the steel, Ni, which also a very expensive alloy element, the adding amount of which is at least half of the amount of Cu, must be added to avoid the cracks. Therefore, replacing Cu with V can greatly reduce the manufacturing cost of the steel.
Ti: A ferritic formation element, which reduces austenitic area significantly. The carbide of Ti, i.e. TiC, is relatively stable, and can inhibit the growth of the crystal grain. Ti, solid solved in austenite, is favourable to improve the hardenability of the steel. Ti can reduce the first type of the temper brittleness, i.e. 250-400° C. temper brittleness. Since the present invention does not need the thermal refining, the adding amount of Ti can be reduced. In the present invention, 0.002-0.040 wt. % of Ti is added, which forms fine carbonitride to precipitate out, thus refining the Bainite battened structure.
Al: Al can increase the driving force of the phase change from austenite to ferrite and can intensively reduce the phase cycle of the austenite. Al interacts with N in the steel to form fine and diffusive AlN, which precipitates out and can inhibit the growth of the crystal grain, thus achieving the purpose of refining crystal grains and improving the low temperature toughness of the steel. Too high content of Al will have an adverse impact on the hardenability and welding performance of the steel. In the present invention, no more than 0.04 wt. % of Al is added to refine crystal grains, improve the toughness of the steel and guarantees the welding performance.
B: B can dramatically increase the hardenability of the steel. In the present invention, 0.001-0.002wt. % of B is added so that one can readily obtain a high intensity bainite structure from steel under a certain cooling conditions.
In a second aspect of the present invention, it is provided a manufacturing method of a steel plate having a low welding crack susceptibility and a yield strength of 800 MPa, which comprises smelting, casting, heating, rolling and cooling procedures, wherein after rolling procedure, the steel is subjected to the cooling procedure without the thermal refining.
In a preferred embodiment, the thickness of the casted continuous casting billet or steel ingot is not less than 4 times of the thickness of the finished steel plate.
In another preferred embodiment, the heating temperature in the heating process is 1050 to 1180° C., and the holding time is 120 to 180 minutes.
In another preferred embodiment, the rolling is divided into the first stage of rolling and the second stage of rolling.
In another preferred embodiment, in the first stage of rolling, the start rolling temperature is 1050 to 1150° C., and when the thickness of the rolled piece reaches twice to three times of that of the finished steel plate, the rolled piece stays on the roller bed until the temperature reaches 800-860° C.
In another preferred embodiment, the Pass deformation rate in the second stage of rolling is 10-28%, and the finish rolling temperature is 780-840° C.
In another preferred embodiment, in the cooling process, the steel plate enters an accelerated cooling device and is cooled at a rate of 15 to 30° C./S to a temperature of 350 to 400° C., followed by air cooling.
In another preferred embodiment, the air cooling is performed by the way of cooling in packed formation or bank cooling.
In the manufacturing method of a steel plate having a low welding crack susceptibility and a yield strength of 800 MPa, the technical control mechanism of the main steps is analyzed as follows:
1. Rolling Process
When the thickness of the rolled piece reaches twice to four times of that of the finished steel plate, the rolled piece stays on the roller bed until the temperature reaches 800 to 860° C. For the steel containing Nb, the non-recrystallizing temperature is about 950 to 1050° C., and it is firstly rolled at a relatively high temperature from 1050 to 1150° C. to produce a certain dislocation density in the austenite, then during the relaxation process of lowering the temperature to roll the billet to 800-860° C., the inside of the austenite crystal grains is subjected to a restoration and statically recrystallization process, thus refining the austenite crystal grains. In the relaxation process, individual precipitation and complex precipitation of carbonitride of Nb, V and Ti occur. The precipitated carbonitride pins the dislocation and subgrain boundary movement, reserves a lot of dislocation in the austenite crystal grains, and provides a lot of nucleation sites for the formation of bainite during the cooling process. Rolling at 800-860° C. greatly increases the dislocation density in the austenite, and the carbonitride precipitated at the dislocation inhibits the coursing of the deformed crystal grains. Due to the precipitating effect caused by deformation, a relatively large Pass deformation will facilitate the formation of finer and more diffusive educts. High dislocation density and fine and diffusive educts provide high density of nucleation sites for bainite, and the pining effect of the second phase particles to the bainite growth interface inhibits the growth and coursing of the bainite battern, which is beneficial for both the intensity and toughness of the steel.
The finish rolling temperature is controlled in the low temperature section of the non-recrystallization zone, and at the same time, this temperature section is close to the transmission point Ar3, i.e. the finish rolling temperature is 780-840° C., and finishing rolling within this temperature range can increase the defects in the austenite by increasing the deformation and inhibiting the restoration, thus providing higher energy accumulation for the bainite phase change while not bringing about too much burden to the roller, suitable for producing thick plate.
2. Cooling Process
After the rolling is complete, the steel plate enters an accelerated cooling device, and cooled to 450 to 500° C. at a cooling rate of 15 to 30° C./s. Rapid cooling speed can avoid the formation of ferrite and pearlite, and the steel plate directly enters the bainite transition area of the CCT curve. The phase change driving force of the bainite can be represented byΔG=ΔGchem+ΔGd wherein ΔGchem is a chemical driving force, ΔGd is a strain stored energy caused by defects. Since rapid cooling speed causes the overcooling of the austenite and increases the driving force of a chemical phase change, ΔGchem should be considered in combination with the strain stored energy ΔGd caused in the rolling process to increase the driving force of the bainite nucleation. Due to the high dislocation density in the crystal grains, the nucleation sites of bainite increase. Considered by combining the thermodynamic and dynamic factors, the bainite can nucleate at a very large speed. Rapid cooling speed enables the bainite transformation to be completed quickly and inhibits the coarsing of the bainite ferrite battern. After exiting from the accelerated cooling device, the steel is cooled in packed formation at 450-550° C. or air cooled in a cold bed to make the carbide of V in the ferrite precipitate more completely, thus enhancing the contribution of the precipitation strengthening to the intensity.
The steel for high intensity mechanical equipment and engineering construction needs high intensity and excellent toughness. A variety of factors will contribute to the intensity, which can be represented by the following formula:σ=σf+σp+σsl+σd wherein σf is fine grain strengthening, σp is precipitation strengthening, σsl is solid solution strengthening, and σd is dislocation strengthening. Thermo-mechanical treatment of the steel plate is usually done by Thermo-mechanical Controlled Rolling and Controlled Cooling Process (TMCP), which refines the microstructures or forms the high intensity structures such as ultra-fine bainite by controlling-deformation rate and cooling rate, thus improving the yield strength of the steel. Modified TMCP and Relaxation Precipitation Controlling (RPC) technology form a stable dislocation network, diffusive and fine second phase particles precipitate out at the dislocation and subgrain boundary, the bainite battern is refined by promoting the nucleation and inhibiting its growth, and a combined action of dislocation strengthening, precipitation strengthening and fine grain strengthening is produced, thus improving the intensity and roughness of the steel. Its principle mechanism is as follows:
The steel plate fully deforms in the recrystallization zone and the deformed austenite produces a high defect accumulation, thus greatly increasing the dislocation density in the austenite. Restoration and recrystallization occurring during the rolling refine the original austenite crystal grains. After being rolled and deformed, dislocation within the crystals will re-arrange during the controlled cooling relaxation. Since a hydrostatic pressure field exits in the edge dislocation, interstitial atom such as B will enrich to the dislocation, grain boundary and subgrain boundary, reduce the dislocation mobility, and finally the high density dislocation caused by the deformation will evolve during the restoration to form a stable dislocation network. During the relaxation, the microalloy elements such as Nb, V, Ti and the like precipitate out at the grain boundary, subgrain boundary and dislocations in the form of carbonitride of different stoichiometric ratios such as (Nb,V,Ti)x(C,N)y and the like. The second phase particles, such as the precipitated carbonitrides, pin the dislocations and subgrain boundary within the crystal grains and stabilize the substructures such as dislocation wall.
Following relaxation, the dislocation density of austenite is increased by the second stage of rolling process. After relaxation, when the deformed austenite is accelerated cooled, the effects of the austenite with dislocation and precipitation formed by relaxation process on the following phase transformation can be interpreted as (different from the circumstance that after deformation, no relaxation occurs and a lot of dislocations disorderly distribute): firstly, a subgrain boundary which has a certain orientation difference is a preferred position for the nucleation, and if a second phase, which has an incoherent interface with the matrix, precipitates out, it will facilitate the new phase nucleation, and, after relaxation, a lot of new phase crystal grain will nucleate within the original austenite crystal grains. Secondly, since after relaxation, a certain amount of dislocations move to the subgrain boundary, which increases the orientation difference between the subgrains to a cartain extent. After the mediate temperature transformed product, such as bainite, nucleates at the subgrain boundary, it is hindered by the forward subgrain boundary during the growth. When the bainite ferrite forms, its phase change interface is daggled by the precipitated second phase carbonitride particles, which inhibits its growth. TMCP plus RPC process forms a high density of dislocation network structure, and the second phase precipitation material points provide a lot of potential nucleation sites for the nucleation of the bainite ferrite, and, the daggling effect of the second phase particles to the moving interface and the evolved subgrain boundary inhibits the growth of the bainite.
Therefore, the manufacturing process of the present invention can play a combined role of promoting the nucleation of the bainite and inhibiting the growth of the bainite, thus refining the final structure.